An overlayer is a layer of adatoms adsorbed onto a surface, for instance onto the surface of a single crystal.[1]

On single crystals

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Adsorbed species on single crystal surfaces are frequently found to exhibit long-range ordering; that is to say that the adsorbed species form a well-defined overlayer structure. Each particular structure may only exist over a limited coverage range of the adsorbate, and in some adsorbate/substrate systems a whole progression of adsorbate structure are formed as the surface coverage is gradually increased.[2]

The periodicity of the overlayer (which often is larger than that of the substrate unit cell) can be determined by low-energy electron diffraction (LEED), because there will be additional diffraction beams associated with the overlayer.[3]

Types

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There are two types of overlayers: commensurate and incommensurate. In the former the substrate-adsorbate interaction tends to dominate over any lateral adsorbate-adsorbate interaction, while in the latter the adsorbate-adsorbate interactions are of similar magnitude to those between adsorbate and substrate.[4]

Notation

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An overlayer on a substrate can be notated in either Wood's notation or matrix notation.[5]

Wood's notation

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Wood's notation takes the form

 

where M is the chemical symbol of the substrate, A is the chemical symbol of the overlayer,   are the Miller indices of the surface plane, R and   correspond to the rotational difference between the substrate and overlayer vectors, and the vector magnitudes shown are those of the substrate (  subscripts) and of the overlayer (  subscripts). This notation can only describe commensurate overlayers however, while matrix notation can describe both.

Matrix notation

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Matrix notation differs from Wood's notation in the second term, which is replaced by the   matrix that describes the overlayer primitive vectors in terms of the substrate primitive vectors:

 , where  

and so hence matrix notation has the form

 

See also

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Citations

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References

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Textbooks

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  • Kolasinski, Kurt W. (2012). Surface Science: Foundations of Catalysis and Nanoscience (3 ed.). Wiley. ISBN 978-1119990352.
  • Attard, Gary; Barnes, Colin (1998). Surfaces. Oxford Chemistry Primers. ISBN 978-0198556862.

Websites

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